Iframe And Existing Id Within

I have an iframe. I load a page within it. The page I load is a php page that I build with a table with the id of 'readmail_table'. My iframe is 'readmail_frame'. Someone clicks on a message title outside of the iframe and I load the contents of the message in the iframe from the database in my table. Sometimes this takes longer than others depending on how large the message is. What I am doing here is letting the contents load then calculating the width and height of the table and then increasing the iframe's size so there are no scrollbars. This works 98% of the time but I ran into an issue where the table does not exist yet in the iframe and the JS code errors out. So what I did then was add a local var, set it to false then had a while loop check for the table and once it finds it, set the var to false and run my code. Again, mixed results. Sometimes this works then others I get that the table id I am looking for is null but my if statement in the while loop is checking to see if != null

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Like I said, most of the time this works exactly how I need it. I just need a fool proof way to determine that this table is in fact on the page. I am not sure why it works sometimes and not others since I am looping until it is there